Combustion irregularities in an internal combustion engine are useful to diagnose because they point to various disorders likely to-affect the operation of the engine.
Patent FR-A-2 689 934 discloses such a method intended more particularly for such diagnostics when an engine is operating at medium and high speed. According to this method, during each combustion phase in any one of the cylinders, collections of time differences ΔtI separating predetermined angular positions of the crankshaft are measured, and then, using a linear combination of the time differences from the set, a “critical parameter” or “index” representative of the variations in the rotational speed of the engine (crankshaft) during this combustion phase is calculated and this index is compared with a default threshold in order to diagnose whether a misfire is occurring.
FR-A-2 718 489 also discloses a method of the type described in FR-A-2 689 934, but improved in such a way as to eliminate disturbances of the diagnostics by variations in the geometry of a “target” used to measure the aforementioned time differences.
Methods of the same type, modified in such a way as to prevent a “backup” of forces, resulting from a vehicle propelled by the engine being driven over a surface of irregular geometry from disturbing the diagnostics, and to do so without using a sensor dedicated to detecting such forces are also known.
The solutions hitherto proposed do, despite everything, have disadvantages which constitute a limitation that it is difficult to accept in the context of a function discerning poor combustion in an engine.
First of all, if the use of an index representative of the variations in the rotation speed of the engine during the combustion phases is in fact appropriate, the parameter best suited to detecting misfires at high engine speeds proves to be sensitive to high-amplitude fluctuations, noise and jolts transmitted typically to the crankshaft through the drive line when a vehicle is in everyday use.
These disturbances may typically be the result of clutch jolts or of driving over a poor road surface. This may in any case lead to an inappropriate discerning of misfires, depending on the intensity of the disturbances, especially at low engine speeds.
Conversely, a parameter that proves to be well suited to detection at low engine speeds is not very sensitive to these high-amplitude fluctuations but does itself have low sensitivity to the speed variations characteristic of an absence of combustion at high speed.
That being the case, the selecting of one parameter from among several according to the conditions of use of the engine, that is to say in “everyday” use, has already been envisaged. However, in this case, each time these conditions of use are altered, a sequence of reinitializing the diagnostics method is needed and may lead to this method being temporarily disabled. The proliferation of these operating/reinitializing sequences around an engine speed or load threshold, when there is a fluctuation or a transient load situation may also interrupt the diagnostics for a relatively long length of time.
Furthermore, the possible use of hysteresis limits the possibilities of practical embodiment of the system and makes calibration complicated.
It may also be noted that the “critical parameters” or “indices” of the methods of the prior art mentioned hereinabove, and the detection algorithms that use them, prove to be effective only under certain engine load and speed conditions and/or only for certain types of combustion fault. Specifically, in this regard, it is possible to draw a distinction between erratic misfires, systematic misfires (affecting one and the same cylinder for example) and various sequences of misfires affecting different cylinders, for example.
Attempts may be made at alleviating these disadvantages by proliferating the diagnostic processes, each process being, tailored to a particular engine operating domain, or to filtering out one particular cause of disturbance of the diagnostics.
However, this approach is cumbersome and expensive, both in terms of process development time for these diagnostic processes and in terms of computation time, when executing these complex processes.